1. Field of the Invention
The subject invention relates to a gas assisted co-extrusion apparatus for forming food products such as cereal or snack food and a method for using the same.
2. Description of the Prior Art
Conventional co-extrusion systems are designed such that a dough material is extruded about a pumpable filling or fluid. Typically, these systems are designed such that the pumpable filling or fluid is supplied to a co-extrusion die via a pipe attached to the die. The pipe is typically attached at a 90° angle to the die face and then takes a 90° turn inside the die in order to align itself with the center of the die orifice through which the food material is being extruded. The food material is extruded over the pipe creating an inner cavity in the food material as the pipe pumps the filling, or fluid into the inner cavity.
One such system is disclosed in U.K. Patent Application No. 2297936A to Frame. The Frame application discloses a die assembly for the extrusion of an expandable product. The die defines a through bore in which the expandable product is fed from a die face. A co-axial gas pipe is supported in the bore and includes a free end which projects outwardly from the die face. The gas pipe disclosed in the Frame application is attached at a 90° angle to the die face and then takes a 90° turn inside the die, specifically in the bore, in order to align itself with the center of the bore through which the expandable product is being extruded. In addition, the gas pipe enters the die adjacent the die face and as such, only travels over a limited portion of the bore. A die outlet is defined at the die face between the through bore and a tubular spacer that is secured to the gas pipe. Frame further discloses the use of a second pipe for depositing a cream into the product. The expandable product is fed over the tubular spacer and out the die outlet. Based on the heat and pressure of the expandable product at the die outlet, as the expandable product exits the die outlet, it expands. Thus, at the die outlet, the expandable product has a wall thickness that is greater than the thickness of the die outlet between the bore and tubular spacer.
Another such system is disclosed in U.S. Pat. No. 6,586,031 to Kelly. The Kelly patent discloses a method for making hollow tube shaped food pellets or half products. Upon further heating, the pellets of Kelly are puffed to produce expanded, shaped snack products with annular cavities. The method of Kelly begins with the step of extruding dough through a extruder having a die insert. Dough from the extruder is passed through the die insert to form a tube-like rope having a wall portion defining a inner cavity extending axially along the tubular rope. Air is introduced into the inner cavity of the rope via an injector mounted within the die insert as the tubular rope exits the die insert. The air is introduced to the inner cavity to support the wall portion of the tubular rope such that the tubular rope extrudate exiting the die insert maintains its shape without being deformed by the subsequent handling of the tubular extrudate by the stretching rollers. Upon exiting the die insert, the dough immediately expands, or pre-puffs, because of the sudden pressure drop (from the pressure inside the co-extrusion die to atmospheric pressure) and the flash boiling water in the dough. The dough is allowed a specified distance to cool after exiting the die insert before it enters a gap formed between an upper roller and a bottom roller. Each roller generally approximates a conveyor belt assembly. The rollers are spaced from one another such that the distance between them equals the diameter of the die insert exit and is therefore slightly smaller than the expanded, pre-puffed rope entering the gap. Accordingly, the rope is compressed as it enters the gap between the rollers. The rollers are operated at a speed greater than the speed at which the rope is being extruded. Accordingly, upon contacting the rope, the rollers stretch the rope axially along the direction in which the rope is being extruded. The stretching preformed by the rollers counteracts the expansion caused by the pre-puffing and produces a rope having a diameter equal to the diameter of the exit of the die insert. The air supplied to the inner cavity of the rope is controlled such that sufficient pressure is applied to the wall portion of the rope to prevent the collapse of the inner cavity as the rope is compressed and axially stretched by the rollers. Upon exiting the rollers, the rope is fed into a cutter which cuts the tubular rope into discrete hollow tube shaped food pellets or half products for further processing.